Improve refreshing of file statinfo

[joelim-work]

I have found a bug where the file statinfo doesn't update when using :cd to change to a new directory that has not already been loaded by lf. The issue happens because of the following check if app.ui.msg == "" which occurs when loading directories asynchronously:

lf/app.go

Lines 407 to 409 in d8b7df8

	if app.ui.msg == "" {
	app.ui.loadFileInfo(app.nav)
	}

This check comes from #994, where the intention was that asynchronous reloads should update the file statinfo, but also not overwrite error messages.

I have added a new variable ui.msgIsStat to track whether the statline is showing file statinfo or an echo/echoerr message, and changed the check to use this variable instead, which should fix the above issue. This also means the workaround in #1149 is not needed anymore.